I had the same problem with my second hayabusa helmet..........it was too small to get over my head and I thought that I would had to widen it. Until I realized that the top of my head, touched the top of the helmet, that's when I realized it was too smal to begin with. Make sure that yours isn't too small too! You want to make sure that there is still a good amount of room so that you can put padding inside so that it won'r change how the helmet sits on your head.
As far as the strengthening alternative.......there are other ways, but some can get a bit expensive. I think that smooth casting can be pretty strong. I know quite a few people use that. I have seen a few people use it on the inside of their pieces. The hayabusa is actually a good piece to use this stuff on because of the spikes and it would be very hard (nearly impossible) to just fiberglass the inside of those!